Destruction or conversion of atmospheric pollutants in industrial gas streams and internal combustion engine exhaust streams has been a long-standing research and development goal. Such atmospheric pollutants include products of incomplete combustion, such as carbon monoxide and unburned hydrocarbons, oxides of nitrogen ["NOx"], and carbonaceous particulate matter ["PM"].
Lean-burning engines, such as diesel engines and lean-burning gasoline or natural gas engines, often emit levels of pollutants above regulatory limits. In response to air quality regulations, vehicle manufacturers employ pollution control devices in internal combustion engine exhaust systems to reduce these emissions. Traditional gasoline engine pollution control devices employ a ceramic honeycomb monolith or a packed bed of pellets having a coating of a noble metal catalyst. Such devices catalyze the reactions of carbon monoxide and unburned hydrocarbons with oxygen, typically at approximately 260.degree. C. to 427.degree. C. (500.degree. F. to 800.degree. F.). Other devices employ catalysts that also catalyze the reaction of oxides of nitrogen. Unfortunately, two factors render such catalytic devices alone insufficient for treating vehicle engine exhaust (especially diesel engine) and similar industrial emissions. First, the catalytic devices are ineffective at destroying PM, which is present in engine gas streams, especially those from diesel engines. Second, the PM and other particulates deposit on the monolith, thereby preventing gaseous constituents from reaching the catalytic material, or possibly deactivating or poisoning the catalyst. In general, conventional three-way-catalysts fail to reduce NOx under lean-burn (that is, oxygen-rich) conditions common to many internal combustion engines.
Internal combustion engines are the subject of regulations limiting NOx emissions. The simultaneous emission limits for both particulate matter and NOx presents a unique problem because the two pollutants typically have an inverse relationship in engine exhaust. Internal combustion engines generally can be configured and tuned to produce an exhaust stream having low PM and high NOx concentrations or, alternatively, high PM and low NOx concentrations. Traditionally, engines that employ oxidation catalyst devices may be adjusted to minimize NOx formation because of the catalysts' inability to reduce NOx. Such adjustments may compromise engine efficiency and performance.
Although not generally employed in reducing NOx emissions from internal combustion engines, various techniques exist for reducing NOx emissions from gas streams in other applications. One technique for reducing NOx emissions is selective catalytic reduction (SCR), which reduces NOx in the presence of a reducing agent, such as of ammonia (NH.sub.3), over a catalyst. Typically, selective catalytic NOx reduction is employed with exhaust stream temperatures in the range of 288.degree. C.-427.degree. C. (550.degree. F.-800.degree. F.). SCR catalysts have the limitations discussed herein above.
Another approach for removing NOx is selective non-catalytic reduction (SNCR), which employs a chemical that selectively reacts, in the gas phase, with NOx in the presence of oxygen at a temperature greater than 621.degree. C. (1150.degree. F.). Chemical NOx reduction agents used in such processes include ammonia (NH.sub.3), urea (NH.sub.2 CONH.sub.2), cyanuric acid (HNCO).sub.3, iso-cyanate, hydrazine, ammonium sulfate, atomic nitrogen, melamine, methyl amines, and bi-urates.